1. Field of the Invention
The present invention relates to an ion-exchange resin holder for recovering silver from rinsing water of a photographic processing apparatus.
2. Description of the Prior Art
To recover silver from the rinsing water of a photographic processor, there exist the following available alternatives: electrolytic recovery, metallic replacement, precipitation and ion-exchange. It has been established that ion-exchange is the most inexpensive and also the most effective technique for reducing the silver content to a rate that is low enough for effluent water.
Ion exchange can be defined as a reversible exchange of ions between a solid (resin) and a liquid (water containing ionised salts). Silver in the rinsing water is in the ionized form of silver complexes, more particularly silver thiosulphate compounds. Conventionally, to recover the silver, a strong base anion exchange resin is used. In this resin, chloride is the mobile ion and it exchanges with the silver thiosulphate to release a chloride ion while capturing the silver complex.
A serious problem encountered with ion-exchange of photographic rinsing water is that gelatin clogs the resin and restricts the liquid flow.
It has been proposed to operate with flow and counterflow through the resin. Rinsing water flows through resin in a first column to the bottom, the silver becoming captured by the resin and the gelatin becoming physically deposited on the resin. From the bottom of this first column, the water is fed through the bottom of a second column where it vigourously agitates the resin and washes out the entrapped gelatin. After some time the flow through the two columns is reversed so that the second column acts as the silver exchange column while the first column is backwashed. This method is disclosed in SMPTE Journal, Vol. 86, No. 2, February 1977, pages 65 to 68. The mentioned system performs well in practice, but is expensive because of the pressurised columns and the valves that are required to reverse the liquid flow.
There has been proposed a silver recovery device for smaller scale film processing installations operating by means of a bed of ion-exchange resin, fluidised by the rinsing water. The device comprises an ion-exchange resin holder with a liquid inlet opening at the bottom, a liquid outlet opening near the top, a pump for maintaining a circulation of rinsing water through the resin holder, and control means for controlling the operation of the circulation pump so that during operation of the processor the pump is continuously operative and during at least part of the inoperative period of the processor the pump is operated intermittently. This device has been disclosed in our pending EU Application 90 202 985.9 entitled: "Silver recovery device".
We have found that the fluid-bed operation raises difficulties with respect to a satisfactory removal of air bubbles entrapped in a resin holder. Entrapped air forms a problem at the start of a fresh resin pack in the installation, but also when operating the device, particularly at the regeneration of the resin when water has to be replaced by a regeneration liquid, e.g. fixer of the processor. Air bubbles within and in particular beneath the fluidised resin bed tend to reduce the effective cross-sectional liquid area of the holder whereby the circulation pump is not capable of attaining its predetermined rate, even in the case of a fresh resin pack. As the resin becomes progressively clogged by gelatin and the like in use, the flow rate decreases further and the efficiency of the device becomes unsatisfactory.
Another problem with fluid bed-type devices is the presence of dead zones in which there is a stagnation of ion-exchange resin and/or an accumulation of gelatin which is conductive to algae growth.